The liver has more biochemical functions than any other organ. It is an essential organ through which absorbed food, medicine and other foreign materials are passed, and the liver has a function of eliminating acquired or innate toxic materials after transforming those toxic materials into water-soluble form. And, it is generally involved in the metabolic functions of various organs. The liver tissues synthesize and supply essential proteins such as albumin for the human body, and at the same time, generate and discharge active materials in vivo. As explained above, the liver is a crucial organ controlling human metabolic functions, in which numerous chemical processes take place. It has been confirmed so far that approximately 500 chemical processes occur in the liver in a short period of time. Hepatocytes are the chief functional cells of the liver, and each individual hepatocyte is 15-30 micron in diameter. The human liver contains approximately 250 billion hepatocytes.
Damage to hepatocytes results in necrosis of the cells. After necrosis, however, hepatocytes are regenerated owing to their excellent innate regeneration ability. Nevertheless, repetition of necrosis and regeneration triggers hepatic fibrosis, and as a result, hepatic cirrhosis, portal hypertension and complications thereby occur.
A series of related diseases all develop after hepatic fibrosis. Hepatic fibrosis is caused by the accumulation of collagen, a fibrogenic substance, in the liver. Liver cells are composed of hepatocytes, sinusoidal endothelial cells (SEC), Kupper cells, and hepatic stellate cells (HSC), and among these 4 types of cells, hepatic stellate cells play the most important role in hepatic fibrosis (American Journal of Physiology. Gastrointestinal & Liver Physiology, 279(1), G7, 2000). Hepatic stellate cells comprise 15% of total liver cells and normally have a function of storing retinoid which is a vitamin A precursor. However, once hepatocytes are damaged, Kupper cells begin to consume the damaged hepatocytes and secrete cytokines (TGF-beta, PDGF, FGF, HGF, PAF and ET-1) to proliferate hepatic stellate cells. The hepatic stellate cells are differentiated into myofibroblasts. Myofibroblasts synthesize collagen, which accumulates in extracellular matrix and leads to hepatic fibrosis. This means the activation of hepatic stellate cells plays an essential role in the development of hepatic fibrosis.
More precisely, the activation of hepatic stellate cells is accomplished in the three following stages—pre-inflammatory stage, inflammatory stage, and post-inflammatory stage.
In the pre-inflammatory stage, hepatocyte damage induces the secretion of wound hormone, a stimulator of hepatic stellate cell proliferation, or proliferation of hepatic stellate cells by reducing arginase, an inhibitor of the cell proliferation. In most cases, alcohol causes the generation of acetaldehyde or lipid peroxide, resulting in the promotion of a matrix gene expression.
In the inflammatory stage, hepatic stellate cells are proliferated by cytokines (TGF-beta, PDGF, FGF, HGF, PAF and ET-1) secreted in the activated Kupper cells and platelets, which are then differentiated into myofibroblasts able to generate fibrocytes (Seminars in Liver Disease, 16(4), 357, 1996; Journal of Hepatology, 26(6), 1220, 1997).
In the post-inflammatory stage, cytokines and growth factors are secreted in completely differentiated myofibroblasts to activate non-differentiated hepatic stellate cells and to secrete extracellular matrices. Myofibroblasts activated and differentiated from hepatic stellate cells synthesize collagen, which is then accumulated in extracellular matrix. Collagen monomer is very unstable and easily decomposed at body temperature, and the decomposed monomers are polymerized to induce hepatic fibrosis (American Journal of Physiology, 264(4 Pt 1), G589, 1993).
Hepatic cirrhosis is attributed to hepatic fibrosis, which is developed by the polymerization of the continuously accumulated collagen, changing the accumulated collagen into insoluble fiber. Hepatic cirrhosis can also be induced by continuing inflammation in the liver accompanying hepatocyte destruction, regeneration and scarring, caused by long-term alcohol abuse, hepatitis, exposure to toxic substances, etc. As a result, the size of the liver is reduced and the surface of the liver becomes bumpy. Severe hepatic cirrhosis is a serious disease causing lethal complications such as portal hypertension, hemorrhage (especially in esophagus and stomach), hepatoma, intoxication by the accumulation of waste matters, coma, etc (N. Engl. J. Med. 350:1646-1654).
Portal hypertension is closely related to the activation of hepatic stellate cells, hepatic fibrosis and hepatic cirrhosis. Myofibroblasts differentiated by the activation of hepatic stellate cells reduce hepatocyte elasticity, and so intrahepatic resistance increases and portal hypertension is developed (Semin Liver Dis 2001; 21:337-349).
Unlike other organs, liver tissue characteristically has a double pathway of blood flow, which is arterial blood with plenty of oxygen flows into liver tissue through the hepatic artery, and venous blood containing nutrients absorbed from stomach or intestines flows in through the hepatic portal vein. The amount of blood flowing in through the hepatic artery is about 400 ml per minute, and the amount of blood flowing in through the hepatic portal vein is about 1200 ml per minute, meaning that ¼ of total blood flowing into the liver takes the road of hepatic artery, while the remaining ¾ takes the road of hepatic portal vein.
Portal blood pressure, similar to other venous pressure, is only about 1/10 of arterial pressure, and this can easily lead to disorders in blood circulation. The repetition of damage to and regeneration of hepatocytes by continuing inflammation results in the accumulation of fibrous materials and the development of regeneration nodes. Regeneration nodes put pressure on the pathway of blood in liver tissue or constrict the blood vessel itself, causing blood circulatory disturbance. While blood flow through the portal vein is not changed, blood flow through liver tissue is decreased by the disorder in blood circulation. As a result, portal blood pressure is increased, causing portal hypertension. The portal vein is a kind of vein without antireflux valve, so blood reflux can occur any time portal blood pressure is increased by circulation disorder, and then the blood seeks a detour in circulation. As a result, collateral vessels in the digestive track (in particular esophagus and stomach) are developed, thereby causing hypersplenism. Collateral vessels are generally developed in low pressure areas such as submucosa of esophagus, anteriolateral abdominal wall, rectum, etc, with consequent symptoms of esophageal varix, ascitic fluid, hemorrhoids and splenic enlargement.
The most common complication of portal hypertension is esophageal varices which need at least 12 mmHg of pressure to be formed. Approximately one third of liver cirrhosis patients show varices in esophagus and stomach, which account for about 30% of causes of death (American Family Physician, 55(5), 1851, 1997). Until now, the factors involved in bleeding by esophageal varices and gastric varices are not fully understood, but the size of varices is believed to be associated with the severity of portal hypertension (Pharmacotherapy: a phathophysiologic approach, 1996).
To treat portal hypertension, surgical operations such as splenectomy or portacaval shunt have been performed to reduce portal bed flow. Medicaments for portal hypertension are exemplified by vasopressin generally used for acute variceal hemorrhage, somatostatin, non-specific beta-adrenergic blocker, alpha-adrenergic blocker, and nitrate preparations. These medicaments decrease portal vein pressure by reducing the arterial flow towards the liver. As a result, the whole portal blood stream to the liver, which is already badly affected, deteriorates even further. Therefore, for many years there has been a need for the development of substances which decrease the portal vein pressure selectively.
It has been proved that phosphodiesterase type-5 (hereinafter referred to as “PDE 5”) inhibitor, already known as an active ingredient in medication for erectile dysfunction, is also effective in treating portal hypertension and diseases related thereto.
For example, the use of PDE 5 inhibitors for the prevention and treatment of portal hypertension is described in PCT/EP2004/006014. Precisely, PDE 5 inhibitors, sildenafil and vardenafil, have preventive and therapeutic effects on portal hypertension and its complications by lowering portal blood pressure through increasing the diameter of the portal blood vessel and portal blood flow.
However, relaxation of the portal vein does not automatically mean the increase of blood flow through the liver and the decrease of portal blood pressure, and in fact, the effect of a specific PDE 5 inhibitor on blood flow through the liver and portal blood pressure is unpredictable.
According to an earlier report investigating the effect of sildenafil on systemic and visceral hemodynamics in experimental cirrhosis models, sildenafil reduces average arterial pressure, causing systemic hypotension, and increases blood flow through mesenteria and portal blood pressure dose-dependently (Liver International, 24(1), 63, 2004; Digestive Disease Week, Abs S1553, 2003). Thus, the researchers who performed the above experiments concluded that additional studies are required to prescribe sildenafil to a cirrhosis patient, because the increase of portal blood pressure by sildenafil might bring bleeding complications.
In the case of liver cirrhosis, splanchnic vascular relaxation by over-production of local NO is observed. According to a report, sildenafil increases the effect of NO, resulting in the decrease of angiomesenteric tonicity and the increase of portal blood flow (Liver International, 24(1):63, 2004; Digestive Disease Week, Abs S1553, 2003).
More or less, metabolic function of liver is reduced in patients with liver disease such as liver cirrhosis, portal hypertension, etc, so that the area under the concentration-time curve (AUC) and the half-life of medicament is increased (Alimentary Pharmacology Therapeutics, 20(1), 29, 2004; Methods and Findings in Experimental and Clinical Pharmacology, 25(8), 625, 2003).
Nevertheless, a patient with chronic liver disease needs long-term administration and multiple prescriptions. If the patient is prescribed medicine having a short half-life, it lowers the rate of patient compliance and hinders effective treatment.
Therefore, studies have been undertaken to prepare a medicine for the treatment of chronic liver disease that maintains its pharmaceutical effect continuously, and increases the rate of patient compliance. And it was reported that when synthetic somatostatin-like octreotide was administered once a day as a sustained release preparation, the effect of reducing portal blood pressure was long lasting (Hepatology Research, 19(2), 108, 2001). Furthermore, when sustained release lanreotide was administered once a day by intramuscular injection to a mouse with portal hypertension induced by hepatoportal sclerosis, peripheral vasodilation and excessive blood circulation were postponed and portal hypertension and visceral congestion, in addition to portal-systemic shunt, were prevented (Journal of Hepatology, 31(3), 482, 1999).
Considering the above problems, there is a need to develop a novel medicine that can increase hepatic blood flow without side effects, reduce portal blood pressure, and increase compliance with long half-life.
The present inventors synthesized a novel compound, pyrazolopyrimidinone derivative 5-[2-propyloxy-5-(1-methyl-2-pyrollidinylethylamidosulphonyl) phenyl]-1-methyl-propyl-1,6-dihydro-7H-pyrazolo(4,3-d)pyrimidin-7-one, and reported its PDE 5 inhibiting effect in a previous study (Korean Patent No. 377,782). Then, the present inventors kept studying on pyrazolopyrimidinone derivative, as a PDE 5 inhibitor, and completed this invention by confirming that the pyrazolopyrimidinone derivative has excellent collagen synthesis inhibitory effect, and can enhance medicinal compliance of chronic liver disease patients since pyrazolopyrimidinone derivative has a long half-life and reduces portal blood pressure.